The application of lubricious coatings to medical devices used for cutting tissue, such as hypodermic needles, surgical needles, and scalpels, has been long recognized as a method for reducing cutting forces and associated pain encountered in using these devices. The most widely recognized materials used for these coatings are silicones. Specifically, mixtures of polydimethylsiloxane (PDMS) and curable amino-functional silicone dispersion are the most commonly used silicone lubricant. Functionally, PDMS serves as a friction reducing lubricant. The amino-functional silicone dispersion cures (crosslinks) the binding of the coating to the medical device's cutting edge. These materials are available from several sources, the best known being Dow-Corning, which markets their PDMS in a range of viscosities as DC-360 Medical Fluid, and their amino-functional silicone dispersion as MDX4-4159. These products have been available for over 25 years, and have generated various methods and coating formulations for specific applications. These methods include single and two step application methods.
Unfortunately, a disadvantage of using these materials in the coating of hypodermic needles is the cure time associated with the dispersion. The dispersion is a member of a family of silicone materials known as Room Temperature Vulcanizates (RTV). In these materials, curing is triggered by exposure of the coating to atmospheric moisture. RTV silicones require small quantities of water to initiate a cure. According to Dow-Corning's product literature, curing requires 7-10 days at ambient conditions. In the prior art, the cure time can be accelerated by temperature, one example of which is the two step method described in U.S. Pat. No. 5,911,711 (the '711 patent). Unfortunately, the acceleration of the cure time disclosed in the '711 patent still inhibits the manufacturing process. While the two step coating process describe in the '711 patent can provide improved product performance by reducing penetration forces, it suffers from the need to have a long delay period between coating steps to allow partial curing.
Another method of using two coat lubricants is described in U.S. Pat. No. 5,985,355 (the '355 patent). In the '355 patent a first “leveling” coat is applied as a base coat to smooth out irregularities on the coating substrate. The base coat is followed by a lubricious top coat. Unfortunately, the two step process of the '355 patent also suffers from the need to have a delay period between coating steps to allow curing. This delay period creates manufacturing problems for products that could benefit from a two step coating.
In the high speed manufacture of hypodermic needles, needles coated by a process without a cure cycle may be exposed to the atmosphere for a brief interval and then are often packaged in a non-permeable package. Unfortunately, the lack of sufficient exposure to atmospheric moisture has been shown to extend the cure time to as much as 4-5 weeks. This extended cure time results in increased lead time manufacturing and supply problems. This increase in manufacturing lead time can potentially produce products, which in a clinical setting do not have optimum lubricant functionality and therefore increase patient discomfort.